The present invention relates to a vacuum switch which comprises an evacuated envelope having therein two contact elements having an electrically insulating gap therebetween, and an electromagnetic coil operative to produce a magnetic field in the region of the gap between the contact elements, the magnetic field being substantially parallel to the width of the gap. A vacuum switch of this kind is known from British patent specification No. 1,258,015.
The known vacuum switch is provided with an electromagnetic coil surrounding the envelope of the vacuum switch, one of the terminals of the coil being connected to a movable contact element of the vacuum switch so that the other stationary element of the switch and the other terminal of the coil can be used to connect the vacuum switch in a current circuit to be protected. The coil is connected in series with two contact elements and when an electric current flows through the vacuum switch, and thus also through the coil, the coil produces an axial magnetic field, i.e., a magnetic field which during current interruption is parallel to the width of the gap between the contact elements and parallel to the arc to be interrupted between the contact elements. The magnetic field increases the current-interruption capability of the vacuum switch. With the known vacuum switch, the coil is continuously energized, i.e., even when the switch is in a closed condition during which the magnetic field is not required. This leads to additional Joule losses in the coil and consequently to an additional temperature rise of the vacuum switch such that the current-carrying capacity of the vacuum switch is lower than required.